The present invention relates to an optical system for fabrication of an integrated circuit of a capacity of 64 M bits to 254 M bits. More particularly, the present invention relates to an optical system for fabrication of an integrated circuit in which an integrated circuit pattern image is projected on a wafer using ultraviolet laser light of a short wavelength having a narrowed spectral band width with the help of an optical system made of quartz glass material.
Heretofore, an optical photolithographic technique, in which a pattern formed on a mask is transferred onto a wafer by illumination, has widely been used as an optical system for fabrication of an integrated circuit, owing to its advantage in cost compared with other techniques, such as those using an electron beam, an X-ray or the like.
While a projection aligner has been developed in a conventional photolithographic technique, said projection aligner using an i-line light of a wavelength of 365 nm emitted from a high pressure mercury lamp for depicting a patterned image with a line width of 0.5 to 0.4 xcexcm, such a projection aligner corresponds to an integrated circuit (IC) of a capacity of 16 M bits to or less.
An IC of 64 M bits to 256 M bits of the first generation requires a resolution of 0.25 to 0.35 xcexcm and an IC of 1 G bits requires a resolution of 0.13 to 0.20 xcexcm. A resolution of 0.35 xcexcm is not achieved by a wavelength of an i-line light and therefore, KrF light is used as a light source. In a region of a resolution better than 0.2 xcexcm, ArF light is used instead of KrF light, especially an ArF excimer laser.
There are various problems in an photolithographic technique using an ArF excimer laser and one of them is a problem associated with an optical material constituting lenses, mirrors and prisms, which is used to construct an optical system for projection.
While an optical material which shows a good transmittance in a wavelength of 193 nm emitted from an ArF laser is substantially limited to quartz glass, especially high purity synthetic quartz glass, ArF light adversely affects quartz glass, damaging it ten times or more than KrF light does.
An improved resistance to excimer laser irradiation of quartz glass is dependent on a hydrogen concentration contained therein, as described in EP-A 401 845. In the publication, it is disclosed that quartz glass may be used to construct an optical system of a projection aligner using a KrF excimer laser as a light source which was able to secure enough resistance to the laser light with a hydrogen concentration of 5xc3x971016 molecules/cm3 or more contained therein.
However, since influence exerted on quartz glass by ArF laser light is extremely greater, compared with KrF laser light, as has been described above. It was found from an investigation into the degree of the damage, such as changes in transmittance and refractive index thereof produced in the bulk of synthetic quartz glass required hydrogen concentration for preventing occurrence of the damage of 100 to 1000 times or more as large as the hydrogen molecule concentration required in the case of a KrF excimer laser light, specifically 5xc3x971018 molecules/cm3 or more.
There are two methods to make synthetic quartz glass incorporating hydrogen molecules. In one of the methods, wherein hydrogen molecules are incorporated into the synthetic quartz glass in atmospheric pressure, adjusting the environment in production, the maximum concentration of hydrogen molecule to be incorporated, is on the order of 5xc3x971018 molecules/cm3. In the other method, wherein hydrogen molecules are incorporated into the quartz glass by a heat treatment in a pressured hydrogen atmosphere, the upper limit of an incorporated hydrogen molecule concentration is the same 5xc3x971018 moleculeslcm3 under a pressure of 10 atm as well, when the pressure is its maximum under Japanese Law of High Pressure Gas Control.
In such circumstances, if a hydrogen molecule concentration of 5xc3x971018 molecules/cm3 or more is desired, it is required that quartz glass is subjected to a heat treatment at a temperature of 1000xc2x0 C. or higher and in a high-pressure hydrogen atmosphere of 100 atm or higher as described in EP-A 483 752.
However, since a heat treatment at 1000xc2x0 C. or higher under a pressure of 100 atm or higher provokes new defects in the bulk of quartz glass, a temperature of the heat treatment preferably is in the range of 200 to 800xc2x0 C. (see Publication of Unexamined Japanese Patent Application No. Hei 6-166528). In the case where a great number of hydrogen molecules, which is 5xc3x971018 molecules/cm3, are incorporated into quartz glass by a heat treatment at a temperature in this range in a hydrogen atmosphere, there arises a fault that it takes very long to incorporate hydrogen molecules in the quartz glass to such a concentration, since a diffusion coefficient of a hydrogen molecule is not so large and in addition, to conduct a heat treatment in a high pressure atmosphere results in not only decrease of homogeneity in refractive index in the bulk of the quartz glass but also generation of strains in the bulk.
Therefore, if quartz glass is subjected to a high-pressure, high-temperature treatment, as a result another heat treatment becomes necessary for readjustment. These treatments make the process to produce quartz glass which is suitable for constituting an optical system of a projection aligner industrially very complex and time-consuming, which in turn makes the quartz glass having a hydrogen concentration of 5xc3x971018 molecules/cm3 or more, a homogeneity in refractive index, a low level of strains and the like, highly expensive.
However even if the quartz glass having hydrogen concentration of 5xc3x971018 molecules/cm3 or more, optical properties, such as a homogeneity in refractive index, a low level of strains and the like, is obtained, a volume constriction causing a change in refractive index of the quartz glass may arise during irradiation by ArF light.
A technique to combine synthetic quartz and fluorite in an optical system used for integrated circuit fabrication has been disclosed in Publication of Unexamined Japanese Patent Application No. Hei 8-78319 (hereinafter referred to as first prior art technique), but the technical concept is essentially different from that of the present invention.
The first prior art technique is to constitute the optical system for exposure with a diffraction optical element having a positive power, a quartz lens having a negative power and a fluorite lens having a positive power, which constitution is to correct a chromatic aberration. In the first prior technique, the chromatic aberration is corrected by a combination of a diffraction optical element and a refracting lens respectively having positive and negative powers, and a fluorite having a positive power, wherein a combination of optical elements, different from one another in optical dispersion corrects chromatic aberration. Such a combination specially realizes an optical system having an image formation property that a secondary spectrum of chromatic aberration is minor and thereby not only can a larger curvature radius of a lens, a larger NA, a larger field of view be realized to give room to improvement on a specification of the optical system but a tolerance of eccentricity in fabrication is also larger to promote easy fabrication.
It is an object of the present invention to provide an optical system for use with ultraviolet light of a short wavelength with a narrowed spectral band width, especially ArF laser light, as illumination in the projection aligner without degrading properties such as durability, optical transmittance and the like with a low cost and ease as a whole.
In the present invention the optical system is constructed of a combination of synthetic quartz glass and fluorite.
Therefore, since the first prior art technique is one in which a quartz glass lens having a negative power and a fluorite lens having a positive power are combined from the view point of solving problems in design and fabrication, it is different from the present invention in that the present invention has the object to prevent degradation in resistance to laser light caused by high output irradiation. Therefore, the prior art is also confirmed to be different in constitution on the basis of the different objects, as mentioned above.
In order to improve resistance to laser light, according to a first aspect of the present invention, an optical system comprises a combination of optical members made of synthetic quartz glass and fluorite, wherein: a crystalline optical member, which is located in a position through which laser light is transmitted at a relatively high light energy density xcex5 (mJ/cm2) (hereinafter referred to as wafer side optical member), is made of fluorite to avoid being broken under irradiation of ArF laser light; and an amorphous optical member, which is located in a position through which laser light is transmitted at a relatively low light energy density xcex5 (mJ/cm2) (hereinafter referred to as light source side optical ember), is made of synthetic quartz glass having a hydrogen molecule concentration in the range of 1xc3x971017 molecules/cm3 to 5xc3x971018 molecules/cm3.
The relatively low hydrogen content of the quartz glass of the amorphous optical member can be simply achieved by doping the quartz glass in atmospheric pressure. As a result it is more easy to establish a high quality of the optical properties of the quartz glass, especially to attach importance to homogeneity of refractive index. The entire optical system achieves high transmittance.
More preferably, the optical system comprises optical members made of a plurality of qualities of synthetic quartz glass and an optical member made of a kind of fluorite, wherein the fluorite has a homogeneity of refractive index xcex94n of 3xc3x9710xe2x88x926/cm or less and a birefringence of 2 nm/cm or less; a first quality of synthetic quartz glass has a hydrogen molecule concentration in the range of 5xc3x971017 molecules/cm3 to 5xc3x971018 molecules/cm3, a homogeneity of refractive index xcex94n of 2xc3x9710xe2x88x926/cm or less and a birefringence of 1 nm/cm or less; and a second quality of synthetic quartz glass has a hydrogen molecule concentration in the range of 1xc3x971017 molecules/cm3 to 5xc3x971018 molecules/cm3, a homogeneity of refractive index xcex94n of 2xc3x9710xe2x88x926/cm or less and a birefringence of 1 nm/cm or less.
That is, while according to the present invention, in a high energy level region of laser light, fluorite having resistance to a change in transmittance of laser light, especially single crystal fluorite due to absolutely no volume constriction over time is observed instead of quartz glass, it is extremely difficult to achieve such optical properties as homogeneity in refractive index, low birefringence and the like for a large diameter optical member used in photolithography when fluorite is used as the material.
If synthetic quartz glass capable of being doped under atmospheric pressure and having high homogeneity is used in a low energy level region of laser light, resistance to a change in transmittance and high homogeneity can be maintained in the entire optical system.
As a relation between fluorite and synthetic quartz glass in terms of a measure of homogeneity such as a refractive index An or a birefringence nm/cm, it is preferred that the homogeneity is set better in the synthetic quartz glass used as an amorphous optical member than in the fluorite used as an crystalline optical member, in other words, a measure of homogeneity is set better in a light source side-optical member than a wafer side-optical member. In a more definite manner, it is preferred that a wafer side-optical member is of single crystal and has a homogeneity of refractive index xcex94n of 3xc3x9710xe2x88x926/cm or less and a birefringence of 2 nm/cm or less, and a light source side-optical member is of synthetic quartz glass and has a homogeneity of refractive index xcex94n of 2xc3x9710xe2x88x926/cm or less and a birefringence of 1 nm/cm or less.
Generally, excimer laser light has a spread in oscillating wavelength and accordingly, unless the spread in oscillating wavelength is restricted to a narrow band width in a monochromatic lens system, a material of whose constituent lenses is only quartz, chromatic aberration arises. Therefore the present invention employs an ArF excimer laser light which is ultraviolet light of a short wavelength having an oscillating wavelength width of 1.5 pm or less (FWHM or full width at half maximum).
The fluorite used as an optical member can secure an internal transmittance per cm of about 98%/cm after irradiation of the pulsed laser light having a wavelength of 193 nm at an energy density per pulse of 50 mJ/cm2 for a pulse count of 1xc3x97106.
In order to improve resistance to laser light, according to a further aspect of the present invention, an optical system for circuit pattern exposure of ArF excimer laser light having a wavelength of 193 nm is provided, wherein the average transmittance of the optical system as a whole is about 98.0%/cm or more, the transmittance of the at least one amorphous optical member is 99.5%/cm or more and the transmittance for the crystalline optical member is 99.8%/cm or more, whereby at least one amorphous optical member belonging to the second quality is disposed in a region through which the laser light is transmitted at an energy density of xcex5xe2x89xa60.1 mJ/cm2, and at least one amorphous optical member belonging to the first quality is disposed in a region, through which the laser light is transmitted at an energy density of 0.1 less than xcex5 less than 0.4 mJ/cm2, and the at least one crystalline optical member is disposed in a region, through which the laser light is transmitted at an energy density of xcex5xe2x89xa70.4 mJ/cm2.
Besides, in order to achieve homogeneity in refractive index in the entire optical system and more particularly, maintain a change in the average refractive index at a low value, the optical members are preferably so arranged in combinations that the optical system comprises an overall optical path length composed of total optical path lengths of the at least one crystalline optical member and of the at least one amorphous optical member, wherein the total optical path length of the at least one crystalline optical member is set at 25% or less of the overall optical path length and the total optical path length of the at least one amorphous optical member belonging to the second quality is set at 50% or more of the total optical path length. Preferably the average variation of refractive index An of the optical system is set at a value of xcex94nxe2x89xa62.0xc3x9710xe2x88x926/cm.